This invention relates to a borehole guide and to a method of boring interpenetrating holes.
In mining or excavating operations in general it is frequently necessary to bore adjacent overlapping holes. This may for example be the requirement where a narrow band of reef is to be extracted from the rock face. It is then essential to be able to drill successive holes accurately in the reef so as to recover the maximum amount of ore. Clearly where a hole is drilled overlapping an already drilled hole there is a tendency, which depends on the degree of overlap, for the later hole to run into the earlier one.
Various devices have been provided to maintain the drill on course, some exterior to the body being drilled, and some located within the preceding hole.
U.K. Pat. No. 865935 describes a tubular guide which is inserted into a first hole and which then forms a guide surface for a second overlapping hole. The device has a limited application in that it can be used only over lengths of up to 5 meters and in addition the overlapping holes are all parallel to one another. The problem exists with this type of device that when long holes i.e. of 20 meters or more are to be drilled it is physically very difficult to locate a guide of this length in a hole and moreover the frictional forces which are to be overcome in locating the guide and removing the guide from the hole are excessive.
German Pat. No. 377989 discloses a wedge shaped deflection piece which permits non-parallel overlapping holes to be drilled. The device is effective however only over a limited length.
Another type of borehole guide which is mentioned extensively in the literature in general makes use of a torpedo or other guide member which travels with the drill and alongside it in a hole which is already drilled. With this type of device long holes can be drilled but they are parallel to one another and no provision is made for the orientation of a second hole to be altered relatively to a first hole. Devices of the type in question are described for example in the specifications of U.K. Pat. No. 1456937, German Pat. Nos. 255979 and 972350, and U.S. Pat. Nos. 2,236,399, 2,308,067, 2,368,511, 3,170,527, 3,874,463 and 3,805,899.